1. Field of the Invention
The present invention relates to a fuel injector for a diesel engine in which a fuel injection rate is made variable.
2. Description of the Prior Art
A generally well-known fuel injector for a diesel engine is an automatic valve, and, as shown in FIG. 3, a needle valve 103 in which a distal end needle valve portion 104 comes into contact with a valve seat 107 disposed in the vicinity of injection ports 106 is provided in a sliding bore 102 provided in a lower portion of the interior of a valve body 101, this needle valve 103 being urged downwardly by a valve spring 108 via seat 109.
Fuel oil fed under pressure from an unillustrated fuel injection pump flows into a fuel passage 110, and the needle valve 103 is subjected to pressure P of fuel oil applied to a lower end of a pressure receiving portion 105 of the needle valve 103 and thus tends to move upwardly. When the force .pi./4 (x.sup.2 -y.sup.2).multidot.P exceeds the force pushing down the needle valve 103, the needle valve 103 moves upwardly, which in turn causes the distal end needle valve 104 to move away from the valve seat 107, causing fuel oil to be injected through the fuel ports 106.
As a result, the pressure receiving area of the needle valve 103 increases from .pi./(x.sup.2 -y.sup.2) to .pi..times..sup.2/ 4, and the pressure of fuel oil is also applied to the lower surface of the needle valve 103. Consequently, the force pushing up the needle valve 103 increases, and the needle valve 103 rises sharply until an upper end 103a of the needle valve 103 collides against an upper end 102a of the sliding bore 102.
A description will be given of this operation with reference to FIG. 4 which shows four plots A to D of certain variables with respect to time. In plot A, the ordinate represents fluctuations of pressure within the fuel passage 110 resulting from the supply of fuel from a fuel injection pump into the fuel passage 110. In plot B, the ordinate represents the net force acting downwardly on the needle valve 103 resulting from the force due to the pressure within the fuel passage tending to push up the needle valve and the opposing force due to the spring 108. Similarly, ordinates of the plots C and D represent the lift of the needle valve 103 and the fuel injection rate respectively.
As described above, the pressure within the fuel passage increases from P.sub.0 to P.sub.1 by the supply of oil from the fuel injection pump, and this pressure is applied to the pressure receiving portion 103a of the needle valve 103. Since the pressure receiving area is .pi./4(x.sup.2 -y.sup.2), force F.sub.1 pushing up the needle valve 103 is P.sub.1 .multidot..pi./4(x.sup.2 -y.sup.2).
Meanwhile, since the force with which the spring 108 pushes down the needle valve 103 is set to the above value F.sub.1, if the pressure within the passage is higher than P.sub.1, the needle valve 103 rises against downwardly pushing force F.sub.1 of the spring 108. At this time, pressure P.sub.1 is also applied to the lower surface of the needle valve 104, so that the force pushing up the needle valve 103 increases sharply to F.sub.2 =P.sub.1 .multidot..pi./4(x.sup.2 -y.sup.2). As a result, the upward movement of the needle valve 103 is accelerated sharply, and the lift of the needle valve 103 from the L.sub.0 to L.sub.1 takes place rapidly until the upper end of the needle valve 103 collides against the upper end 102a of the sliding bore 102. In the drawing, a time interval between T.sub.0 to T.sub.1 is ascribable to a delay in acceleration due to the mass of the needle valve.
The downwardly pushing force of the spring 108 increases from F.sub.1 to F.sub.3 owing to the lift of the needle valve 103. At this time, however, the force pushing up the needle valve 103 is greater than the downwardly pushing force F.sub.3 of the spring 108, as indicated by the curved solid line in plot B of FIG. 4, so that the needle valve 103 maintains full lift.
As the injection of fuel decreases, the pressure within the passage 110 falls to P.sub.2, which balances the F.sub.3 with which the spring 108 pushes down the needle valve 103.
With a further decline in the pressure within the fuel passage, the needle valve 103 is pushed down by the force of the spring 108, and when the pressure falls past P.sub.3 at time T.sub.2, the force due to this pressure no longer overcomes the aforementioned force F.sub.3, so that the needle valve 103 closes (and its lift becomes L.sub.0). Accordingly, the needle valve 103 closes when the pressure in the fuel passage drops to ##EQU1## and the needle valve 103 opens when the pressure reaches ##EQU2## Since P.sub.2 &lt;P.sub.1, the fuel injection rate is slower during valve closing than valve opening.
In actuality, the needle valve 103 being of finite mass closes not at time T.sub.2 but at time T.sub.3, since a delay due to its acceleration occurs. During this delay, the pressure within the fuel passage 10 drops further to P.sub.4. Accordingly, the fuel injection rate which is proportional to the pressure within the fuel passage inevitably drops towards the end of the fuel injection period, as shown in plot D in FIG. 4.
In addition, after the opening of the needle valve portion 104, the injection ports 106 serve as a throttle when fuel is injected, so that it is difficult to set the port diameter. For instance, if the port diameter is set in such a manner as to display optimum performance during medium speed of the engine, the maximum pressure of fuel injection becomes excessively low at low speed in which the rate of fuel supply from the fuel injection pump is low, whereas said maximum pressure becomes excessively high at high speed.
As described above, fuel injected at a high fuel injection rate at the beginning of injection is burnt suddenly within a combustion chamber of the diesel engine and hence generates a sudden increase in pressure. This results in combustion noise due to so-called diesel knock, and also brings about a rise in combustion maximum pressure and a resultant rise in the combustion temperature, with the result that emission of harmful NOx is liable to occur.
In addition, a decline in the fuel injection rate at the end of injection, a resultant increase in the fuel injection period, and the enlargement of fuel droplets caused by a decline in the injection pressure result in the so-called after burning phenomenon. This not only results in the occurrence of harmful black smoke due to incomplete combustion and CO and hydrocarbon emission but also causes the heat efficiency to decline.
To cope with this problem, an attempt has been made to shorten the delay in acceleration by reducing the mass of the needle valve 103, but it has not led to an overall improvement in performance.
In addition, in connection with the throttling by the injection ports 106, the decline in injection pressure at low engine speed enlarges atomised fuel droplets and lowers the combustion efficiency, while, at high engine speed, the injection pressure becomes too high, which increases the stress in the fuel injection pump and results in an excessively large power absorption by the fuel injection pump. Accordingly, since resulting losses surpass the advantage of improved combustion, it follows that no overall improvement in heat efficiency can be attained.